1. Field of the Invention
The present invention relates to passive magnetic bearings, and more specifically, it relates to techniques for stabilizing such bearings as the bearing rotates at a rate that is less than the transition speed where the bearing is dynamically stable.
2. Description of Related Art
In the employment of ambient-temperature passive magnetic bearings based on the teachings of U.S. Pat. No. 5,495,221, “Dynamically Stable Magnetic Suspension/Bearing System,” incorporated herein by reference, it is required that provision be made for stabilizing the system against displacements from equilibrium when the system is at rest or when it is being brought up through the “transition speed,” the rotation speed above which the stabilizer elements become effective. As described in the cited patent, this requirement is to be fulfilled by employing a mechanical bearing that becomes decoupled above the transition speed. One method of achieving this result is to rely on centrifugal forces to accomplish the decoupling. An example of such a system is shown in FIG. 16 of the cited patent.
Motor and generator armatures, flywheel rotors, and other rotatable components have conventionally been supported and constrained against radially and axially directed forces by mechanical bearings, such as journal bearings, ball bearings, and roller bearings. Such bearings necessarily involve mechanical contact between the rotating element and the bearing components, leading to problems of friction and wear that are well known. Even non-contacting bearings, such as air bearings, involve frictional losses that can be appreciable, and are sensitive to the presence of dust particles. In addition, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum environment.
The use of magnetic forces to provide a non-contacting, low friction equivalent of the mechanical bearing is a concept that provides an attractive alternative, one which is now being exploited commercially for a variety of applications. All presently available commercial magnetic bearing/suspension elements are subject to limitations, arising from a fundamental physics issue, that increase their cost and complexity. These limitations make the conventional magnetic bearing elements unsuitable for a wide variety of uses where complexity-related issues, the issue of power requirements, and the requirement for high reliability are paramount.
The physics issue referred to is known by the name of Earnshaw's Theorem. According to Earnshaw's Theorem (when it is applied to magnetic systems), any magnetic suspension element, such as a magnetic bearing that utilizes static magnetic forces between a stationary and a rotating component, cannot exist stably in a state of equilibrium against external forces, e.g., gravity. In other words if such a bearing element is designed to be stable against radially directed displacements, it will be unstable against axially directed displacements, and vice versa. The assumptions implicit in the derivation of Earnshaw's Theorem are that the magnetic fields are static in nature (i.e., that they arise from either fixed currents or objects of fixed magnetization) and that diamagnetic bodies are excluded.
The almost universal response to the restriction imposed by Earnshaw's Theorem has been the following: Magnetic bearing elements are designed to be stable along at least one axis, for example, their axis of symmetry, and then external stabilizing means are used to insure stability along the remaining axes. The “means” referred to could either be mechanical, i.e., ball bearings or the like, or, more commonly, electromagnetic. In the latter approach magnet coils are employed to provide stabilizing forces through electronic servo amplifiers and position sensors that detect the incipiently unstable motion of the rotating element and restore it to its (otherwise unstable) position of force equilibrium.
Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to stably levitate that magnet. These bearing types utilize the flux-excluding property of superconductors to attain a stable state, achieved by properly shaping the superconductor and the magnet so as to provide restoring forces for displacements in any direction from the position of force equilibrium. Needless to say, magnetic bearings that employ superconductors are subject to the limitations imposed by the need to maintain the superconductor at cryogenic temperatures, as well as limitations on the magnitude of the forces that they can exert, as determined by the characteristics of the superconductor employed to provide that force.
The magnetic bearing approaches that have been described represent means for creating a stable situation in the face of the limitations imposed by Earnshaw's Theorem. The approach followed by the first one of these (i.e., the one not using superconducting materials) is to overcome these limitations by introducing other force-producing elements, either mechanical, or electromagnetic in nature, that restore equilibrium. The latter, the servo-controlled magnetic bearing, is usually designated as an “active” magnetic bearing, referring to the active involvement of electronic feedback circuitry in maintaining stability.
The elements described referred to above, in some cases with already known elements, to levitate a rotating system that is maintained in a state of dynamic equilibrium without the use of active feedback circuitry. The avoidance of the instability predicted by Earnshaw's Theorem comes about by a combination of satisfying, for the system as a whole, some well-defined stability criteria, together with the employment of dynamic effects. That is, the system must be passively stabilized above a low critical speed. Below this speed conventional elements, such as ball bearings, may be used to maintain stability, with centrifugally activated means provided to disengage these mechanical elements for speeds higher than the critical speed. It is desirable to provide alternate means for maintaining stability at speeds below the critical speed. The present invention provides such alternate means.